In recent years, in line with the rapid diffusion of the Internet, as one of the challenges involved with increasing the transmission capacity of optical transmission networks wavelength division multiplexing (WDM) has been achieved in which a plurality of signals having different wavelengths are multiplexed for transmission over a single fiber. Optical amplifiers, (e.g. erbium doped fiber amplifiers: EDFA) are essential for WDM systems, however as optical amplifiers normally have wavelength-dependent gain problems arise in connection with saturation in receiver and a deteriorating S/N ratio.
One means employed to resolve such problems is to provide a variable optical attenuator (VOA) at the input side of an optical multiplexer in a WDM system and adjust the signal level at each wavelength. To achieve this, it is preferable to have an array type VOA that uses VOA for each wavelength via a plurality of VOA arranged in parallel. Generally, the features of small input loss, non-polarization dependency, non-wavelength dependency and a wide variable range of attenuation quantity are desirable in a VOA. In the case of an array type VOA, low crosstalk between neighboring VOA in the array, low cost, compact size and low power consumption are also desirable.
Non-mechanical type VOA's that fulfill these requirements operate using electro-optical effects, electromagnetic optical effects and thermo optical effects (changes in refractive index due to temperature). In recent years there has been a focus on optical polymer materials in which changes in index of refraction in response to temperature changes are substantial, for providing a material for a VOA using thermo optical effects (thermo optical type VOA).
In comparison to silicon dioxide (SiO2: silicon), the thermo optical coefficient of optical polymer materials has a number of digits before decimal point greater than that of silicon dioxide by 1 and the rate of thermal conductivity is lower thereby enabling lower energy consumption and lower-cost production. Optical polymers such as organic and inorganic polymers derived from the sol-gel method, polymide resins, epoxy resins and acrylic resins and the like have a negative optical effect in which index of refraction decreases as temperature rises.
In terms of production of a thermo optical type VOA, if an optical polymer material having these effects is used formation of a thin-film by the spin coating method is easy, enabling production processes to be performed at low temperatures, thus there is no necessity to limit the size of a substrate produced so a large area substrate can be produced and the layering of clad layers and core layers that comprise a thermo optical type VOA can be achieved more easily. Further, the spin coating method is not the only method available but a variety of different production methods can be employed bringing further expectations of increased scales of production and further cost reductions.
FIG. 1 is a skeleton plan view showing the configuration of a conventional thermo optical type VOA. In reference to this conventional thermo optical type VOA, refer to Japanese Unexamined Patent Application Publication No. 2002-162654, in the Nov. 23, 2000 issue of Electronics Letters, Vol. 36 No. 24 pages 2032–2033 and the Apr. 26, 2001 issue of Electronics Letters, Vol. 37 No. 9, pages 587–588.
In the thermo optical type variable optical attenuator 20 shown in FIG. 1, once power flows from the power supply 25 a heater 24 heats and the temperature of a heated part of a multi-mode optical waveguide 23 positioned under the heater 24 rises. As this happens, the refractive index of the heated part falls due to the negative optical properties of the optical polymer material comprising the thermo optical type variable optical attenuator 20. Accordingly, incident light φ10 (optical power P10) propagated along single mode optical waveguide 21 is conveyed to a tapered part 22, thereafter, this incident light φ10 excites higher order mode lights φ1 and φ2 at the above-mentioned heated part of the multimode optical waveguide 23. These excited, high order mode lights φ1 and φ2 are diffused around a multi-mode optical waveguide 23, optical leakage occurs and as a result, the output power (P11) of the single mode optical waveguide 27 attenuates (in other words P11<P10).
When an array type variable optical attenuator is configured consisting of a plurality of the above described thermo optical type variable optical attenuators 20 arranged in parallel, light the from each of the multimode waveguides couples with light in adjacent waveguides giving rise to problems due to variable range of attenuation quantity and crosstalk between adjacent waveguides (e.g. where the distance between adjacent waveguides is 250 μm crosstalk is approximately 34 dB).